(1) Field of the Invention
The present invention relates to a non-aqueous secondary cell in which lithium or lithium alloy is used as the active material for the negative electrode, and particularly to an improvement in the positive electrode.
(2) Description of the Prior Art
Molybdenum trioxide, vanadium pentoxide, titanium and niobic sulfide have been proposed as the active material for the positive electrode of this type of secondary cell, but these substances have not been to practical use to date.
For the positive electrode of the non-aqueous primary cell, on the other hand, manganese dioxide and carbon fluoride are known to be typical examples of active material and are actually employed for the purpose. Manganese dioxide in particular has the advantages of being excellent in storage characteristics, abundant in the earth and inexpensive.
As the crystal structure of manganese dioxide suited for the positive electrode, .gamma.-.beta.MnO.sub.2 heat-treated at temperatures of 250.degree.-350.degree. C. has been proposed as in Japanese Patent Publication No. 49-25571. This .gamma.-.beta.MnO.sub.2, however, is unsatisfactory in reversibility and has the problem of lowering charge and discharge characteristics. The reason will be explained with reference to FIGS. 1a-1e of the accompanying drawings showing X-ray diffraction patterns.
FIG. 1a shows a diffraction pattern prior to charging and discharging. FIGS. 1b and 1c show diffraction patterns at a 10th discharge and charge, respectively. Compared with the pattern of FIG. 1a, it will be seen that the patterns of FIGS. 1b and 1c show the angles of diffraction shifting to a lower side and the peaks becoming less sharp also after charging. These trends are more conspicuous and the peaks are almost leveled out in the patterns at a 100th discharge and charge shown in FIGS. 1d and 1e, respectively. It may be deduced from the above that a repetition of charges and discharges results in widening of the bond length between manganese and oxygen and in loosening of the crystal structure of manganese dioxide. Consequently, the manganese dioxide has poor reversibility and charge and discharge characteristics.
This applies also to .beta.-MnO.sub.2 heat-treated at temperatures of 350.degree.-430.degree. C. as disclosed in U.S. Pat. No. 4,133,856.
The active material for the positive electrode may comprise .delta.-manganese dioxide having a laminar structure or .alpha.-manganese dioxide having a structure including larger channels than .gamma.-.beta. and .beta.-manganese dioxides. This is considered to improve the reversibility of the non-aqueous secondary cell since the spaces are then increased for doping and undoping lithium ions.
.delta.- and .alpha.- manganese oxides contain potassium ions and ammonium ions in their structure. These ions elude into the electrolyte during the charging and discharging, thereby to greatly deteriorate the charge and discharge characteristics.